What’s the point of making the trial instance available to me, but not allowing me to assign myself a license? Can anyone please help me get a trial? Ideally I hope that I can get a long-term instance, so that I don’t have to configure the integration options every time I get a new trial instance.

Jason: Computer says “Yes”?

Jason “I can make a kettle talk to CRM” Lattimer to the rescue:
On your O365 user under Licenses, look at the Microsoft Dynamics CRM Online Professional license, you should be able to expand it and under it you’ll see 3 different selections. Uncheck the option for Microsoft Dynamics Marketing Sales Collaboration.